Routers and bridges link two or
more individual Local Area Networks (LANs) to create an extended-network
LAN or Wide Area Network (WAN).

Routers
• Link networks using different network identities.
• Transmit only the data needed by the final destination across the LAN.
• Examine and rebuild packets without passing errors on to the next LAN.

A router stores and forwards data packets—each of which
contains a destination and source network address—from one LAN or WAN to
another. Routers are "smarter" than bridges, because they find the best
route for all the data sent to them by the previous router or the end
station of the LAN.

Routers operate on the third layer of the OSI Model, the
Network-Control Layer. Rather than passing packets based on the Media
Access Control (MAC) Layer addresses (as bridges do), a router examines
the packet's data structure and determines whether or not to forward it.
This determination is made based on the network information within the
packet.

Once the router determines where the packet should be
sent, it finds the fastest route to send the data to its destination. The
router also has to send this data in the most appropriate format for
transferring information. That means it may repackage or break the data
into smaller pieces than the receiving destinations can handle.

Routers don't have a bridge's ability to learn
addresses, so they have to do more data processing than bridges do.
Routers also have to be aware of the network protocols they serve and
often have more complex installation and configuration requirements.

Occasionally a router encounters a protocol it doesn't
understand. If this occurs, it typically just drops the packet. Some
protocols are considered "nonroutable"—they don't define any network
information in the data packet.

However, most routers have bridging capabilities. They
can bridge nonroutable protocols by checking a packet's destination
address and simply sending protocols through, so you don't lose
information. The most popular "routable" protocols are IPX™/SPX, TCP/IP,
and AppleTalk®. If a router doesn't have bridging capabilities, it
automatically discards any packets it encounters with nonroutable
protocols, such as NetBIOS, LAT™, or SNA.

Bridges
• Connect two parts of the same network.
• Read only the destination address of each Ethernet packet or Token Ring
frame for maximum speed and efficiency.

A bridge connects two LAN segments into one larger
continuous LAN. So how does a bridge operate? Unlike routers, every bridge
builds an internal list of addresses of the attached network devices on
both sides of it.

When a bridge sees a packet, it checks the packet's
address against its internal list. If the destination address is on the
opposite segment or if the bridge doesn't have the address logged, the
bridge forwards the information.

Bridges operate at the Data-Link Layer of the OSI Model.
They can distinguish between local and remote data, so data traveling from
one workstation to another in the same segment doesn't have to cross the
bridge.

Bridges operate on MAC-Layer addresses. They're protocol
independent, so they transfer data between workstations without having to
understand the protocol. That means they require little or no
configuration.

The OSI Model for Open
Systems Interconnection.
In 1978, the International Standards Organization (ISO) created a
universal standard for exchanging information between and within networks
and across geographical boundaries. This standard for network architecture
is the seven-layer model for Open Systems Interconnection (OSI), which has
encouraged conformity in designing communications networks and controlling
distributed processing.

Manufacturers are developing intelligent computers and
equipment, and numerous private and public data networks have been created
to connect it. But communication among these distributed systems and
networks requires a standard approach to network design, one that defines
the relationships and intersections between network services and functions
via common interfaces and protocols.

The layered approach to network architecture stems from
the operating system (OS) design. Because of their complexity, most
computer OSs are developed in sections, each of which has a particular
function. This makes it simpler to refine each section to meet its
functional goal. Ultimately, all sections are integrated to provide
complete capabilities and services with a smooth-running OS.

The same is true in designing networking systems. A
network architecture specifies a hierarchy of independent layers that
contain modules for performing defined functions.

This translates into a set of rules that defines the way
participating network nodes must interact to communicate and exchange
information. The OSI Model defines standard relationships between the
hardware and software in today's complex computer systems.

The Physical Layer defines the
electrical and mechanical aspects of interfacing to a physical medium
for transmitting data, as well as setup, maintenance, and disconnection
of physical links. When implemented, this layer includes the software
driver for each communications device, plus the hardware
itself—interface devices, modems, and communications lines.

The Data-Link Layer establishes an
error-free communication path between network nodes over the physical
channel, frames messages for transmission, checks the integrity of
received messages, manages access to and use of the channel, and ensures
the sequence of transmitted data.

The Network-Control Layer addresses
messages, sets up the path between communicating nodes, routes messages
across intervening nodes to their destinations, and controls the flow of
messages between nodes.

The Transport Layer provides
end-to-end control of a communication session once the path has been
established, which enables the reliable and sequential exchange of data
independent of both the systems that are communicating and their
locations in the network.

The Session Layer establishes and
controls system-dependent aspects of communication sessions provided by
the Transport Layer and the logical functions running under the OS in a
participating node.

The Presentation Layer translates and
converts transmitted encoded data into formats that can be understood
and manipulated by users.